Re: [PATCH v5 04/13] mm: Add readahead address space operation

[Matthew Wilcox <willy@infradead.org>]

On Tue, Feb 11, 2020 at 03:52:30PM +1100, Dave Chinner wrote:
> > +struct readahead_control {
> > +	struct file *file;
> > +	struct address_space *mapping;
> > +/* private: use the readahead_* accessors instead */
> > +	pgoff_t start;
> > +	unsigned int nr_pages;
> > +	unsigned int batch_count;
> > +};
> > +
> > +static inline struct page *readahead_page(struct readahead_control *rac)
> > +{
> > +	struct page *page;
> > +
> > +	if (!rac->nr_pages)
> > +		return NULL;
> > +
> > +	page = xa_load(&rac->mapping->i_pages, rac->start);
> > +	VM_BUG_ON_PAGE(!PageLocked(page), page);
> > +	rac->batch_count = hpage_nr_pages(page);
> > +	rac->start += rac->batch_count;
> 
> There's no mention of large page support in the patch description
> and I don't recall this sort of large page batching in previous
> iterations.
> 
> This seems like new functionality to me, not directly related to
> the initial ->readahead API change? What have I missed?

I had a crisis of confidence when I was working on this -- the loop
originally looked like this:

#define readahead_for_each(rac, page)                                   \
        for (; (page = readahead_page(rac)); rac->nr_pages--)

and then I started thinking about what I'd need to do to support large
pages, and that turned into

#define readahead_for_each(rac, page)                                   \
        for (; (page = readahead_page(rac));				\
		rac->nr_pages -= hpage_nr_pages(page))

but I realised that was potentially a use-after-free because 'page' has
certainly had put_page() called on it by then.  I had a brief period
where I looked at moving put_page() away from being the filesystem's
responsibility and into the iterator, but that would introduce more
changes into the patchset, as well as causing problems for filesystems
that want to break out of the loop.

By this point, I was also looking at the readahead_for_each_batch()
iterator that btrfs uses, and so we have the batch count anyway, and we
might as well use it to store the number of subpages of the large page.
And so it became easier to just put the whole ball of wax into the initial
patch set, rather than introduce the iterator now and then fix it up in
the patch set that I'm basing on this.

So yes, there's a certain amount of excess functionality in this patch
set ... I can remove it for the next release.